Thermo-acoustic phenomena manifest in lean premixed flames of gas turbines as pressure oscillations at distinct frequencies characteristic of the burner design and its operation. They can lead to early materials aging or even severe damages. Therefore, a thorough understanding of the processes responsible for these instabilities, i.e. the coupling between the unsteady heat release and the pressure fluctuations, is crucial. In order to study these instability modes, phase-locked 2-D OH laser-induced fluorescence (LIF) measurements have been performed. The fluorescence experiments were carried out on a test rig equipped with a commercial 700 kW burner and a combustion chamber of UV transparent quartz, using a pulsed Nd:YAG/dye laser system and an intensified CCD camera for detection. Intensity variations in the integral OH LIF signal of up to +- 15% over one oscillation period are observed for peak sound pressure of 6 mbar and more. In addition, the phase-averaged position of the flame zone varies in axial direction, i.e. the main flow direction. The outer part of the flame zone, close to the combustor walls, shows much stronger oscillations than the central part. The findings indicate two counterpropagating recirculation zones - one in the center and one close to the tube walls - in agreement with CFD calculations and water channel experiments.
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